1. Field of the Invention
The present invention relates to a soldering technique in electric and electronic parts or circuits using no lead which is one of environmental injurious materials. Particularly, it relates to a method for producing a tin-silver alloy plating film required to have solderability, the tin-silver alloy plating film, and a lead frame for electronic parts which is provided with the plating film.
2. Description of the Prior Art
Recently, environmental problems have been regarded as important, and, as to the parts used for IC packages, it has been attempted to use materials containing no injurious materials for environments.
Of materials used for lead frames for electronic parts, one of the materials especially harmful for environments is lead used in solders. When allowed to stand, lead dissolves out of the solder and exerts a harmful influence upon the human body and living organisms. For this reason, development of solders or solder pastes using no lead has been tried in the electronic industries.
One example of conventional lead frames for electronic parts will be shown below. FIG. 3 is a plan view of a general lead frames for electronic parts, and FIG. 4 is a sectional view of the lead frame for electronic parts shown in FIG. 3 on which is mounted a semiconductor device. In FIG. 3 and FIG. 4, 1a indicates a general lead frame for electronic parts, 2 indicates an outer lead portion, 3 indicates an inner lead portion, 4 indicates a pad, 5 indicates a tie bar portion, 9 indicates a semiconductor chip as a semiconductor device, 10 indicates an adhesive, 11 indicates an electrode pad, 12 indicates a wire, and 13 indicates a molding resin.
The lead frame for electronic parts which has the above construction will be explained.
In FIG. 3, around the pad 4 on which the semiconductor chip 9 is mounted, a plurality of inner lead portions 3 are disposed apart from the pad 4, and the inner lead portions 3 are connected to the outer lead portions 2 through the tie bar portions 5. The lead frame 1a for electronic parts which has the shape as shown in FIG. 3 is obtained by subjecting a plate-like material comprising a Cu alloy, an Fexe2x80x94Ni alloy or the like to pressing or etching treatment. Moreover, the pad 4 and the inner lead portion 3 are subjected to partial plating of about 3-5 xcexcm with a noble metal such as Ag for inhibiting the oxidation.
Generally, the semiconductor chip 9 is mounted on the lead frame 1a for electronic parts in the following manner. That is, as shown in FIG. 4, the semiconductor chip 9 is die bonded on the pad 4 using the adhesive 10. The electrode pad 11 previously formed on the semiconductor chip 9 is wire bonded to the inner lead portion 3 with wire 12 such as of Au, Al or Cu to perform electrical connection. Thereafter, sealing of them including the wire bonded part is carried out with molding resin 13 such as an epoxy resin. Then, the outer lead portion 2 is subjected to plating with a solder such as a tin alloy to impart solderability. After the tie bar portion 5 is cut, flash removal is carried out and then the outer lead portion 2 is bent to accomplish the formation of a resin sealed semiconductor device. The resulting resin sealed semiconductor device is mounted on an outer equipment substrate such as printed circuit board, and a desired electronic equipment is formed by soldering the outer lead portion 2 and the necessary wiring on the substrate.
As substitutes for lead-containing solders, lead frames for electronic parts the whole surface of which is plated with palladium have been put to practical use. However, use of palladium as a single substance causes deterioration in wettability with the solder when heat is applied at the time of die bonding or wire bonding, and thus there is a problem in reliability of soldering conducted for mounting. Therefore, there is proposed a lead frame for electronic parts which is thinly plated with gold as a protective film on the surface of palladium. However, when the outermost surface is subjected to flash plating with gold, the adhesion between the molding resin and the lead frame for electronic parts is deteriorated, and, therefore, it is necessary to use molding resins which are improved in adhesion to gold, but high in cost. Furthermore, countries supplying palladium are limited and short supply thereof causes increase of cost and the cost further increases due to the use of gold as a protective film.
Furthermore, in the case of the lead frames for electronic parts the whole surface of which is plated with palladium, flashes are apt to be formed at the step of sealing the semiconductor device with a molding resin in fabrication of the semiconductor device, and a step of removal of the flashes is necessary, which causes increase of cost. Moreover, in the case of the lead frames for electronic parts the whole surface of which is plated with palladium, a great difference in potential is produced between palladium and a metal which is a substrate of lead frame. Therefore, nickel or palladium-nickel alloy must be allowed to be present between the palladium and the substrate. In this case, if nickel or a nickel alloy or iron or an iron alloy is used as the substrate, corrosion occurs, and, hence, copper or a copper alloy must be used as the substrate at the present.
As a solder or solder paste using a material other than palladium and using no lead, it has been proposed to form a plating film using a solder or solder paste using no lead with addition of a metal such as indium, bismuth or zinc in place of lead used in tin-lead based solders.
In addition, as solders or solder pastes for reflow soldering, there are proposed ternary or quaternary alloys containing two or more metals in addition to tin. However, as solders for plating, it is difficult to control the precipitation composition of the ternary or quaternary alloy in plating solution at the time of electroplating, and binary alloys comprising tin and another metal are mostly used, but alloys comprising tin and indium are difficult to put to practical use because indium is high in cost. Alloys comprising tin and bismuth have low melting points, but are hard and brittle and, hence, inferior in workability. Therefore, they can hardly be used for lead frames which are subjected to bending. Moreover, they are inferior in solder wettability and low in bonding strength and thermal fatigue strength, and bismuth is apt to precipitate at the interface to cause the lift-off phenomenon that IC lifts from the solder at the time of surface mounting. Those which comprise tin and zinc have melting point close to that of tin-lead based solder, and cost of zinc is low, but since zinc is readily oxidized in the air, when the alloys are subjected to thermal hysteresis in the step of fabrication of semiconductor devices, resulting in deterioration of solder wettability.
Therefore, recently, alloys comprising tin to which silver is added are proposed for solders or solder pastes or solder plating film using no lead.
The object of the present invention is to provide a method for producing a tin-silver alloy plating film which does not contain lead which is one of environmental harmful materials and which is superior in solder wettability and can improve solderability, a tin-silver alloy plating film produced by the method, and a lead frame for electronic parts having the film.
In order to attain the object of the present invention, according to the method for producing a tin-silver alloy plating film of the present invention, a tin-silver alloy plating film is formed on a plate-like material such as a lead frame for electronic parts or on a terminal of electronic parts and then the surface of the tin-silver alloy plating film is subjected to a heat treatment.
Temperature of the heat treatment is preferably 70-210xc2x0 C., more preferably 110-170xc2x0 C., most preferably 135-150xc2x0 C. By this heat treatment, the crystal phase of the tin-silver alloy plating film changes from the two phases of Sn phase and Ag4Sn phase to the three phases of Sn phase, Ag4Sn phase and Ag phase, and an oxide layer is hardly formed on the surface and thus solder wettability is improved.
In the case when an oxide layer is formed by the heat treatment, the oxide layer is preferably is subjected to etching treatment.
Furthermore, according to the method of the present invention, the surface of the tin-silver alloy plating film is subjected to a cleaning treatment with a solution containing a phosphoric acid compound and a carboxylic acid compound.
By carrying out the above cleaning treatment, a phosphorus compound such as P, PO2, or PO3 is present in the vicinity of the surface of the tin-silver alloy plating and this compound inhibits formation of an oxide layer on the surface. Here, xe2x80x9ca phosphorus compound is present in the vicinity of the surface of the platingxe2x80x9d means that the phosphorus compound is present on the surface of the plating and in the plating layer within 10 xc3x85 from the surface. As preferred phosphoric acid compounds used for the cleaning, mention may be made of phosphoric acid, phosphates such as sodium diphosphate and disodium hydrogenphosphate, phosphorous acid, phosphites such as sodium phosphite and sodium hydrogenphosphite, pyrophosphoric acid and pyrophosphates such as sodium pyrophosphate. As preferred carboxylic acid compounds used for the cleaning, mention may be made pyrrolidine-2-carboxylic acid, hydroxy-L-proline and ethane 1,2-dicarboxylic acid.
Content of silver in the tin-silver alloy plating film is preferably 1-8 wt % for inhibiting the formation of whiskers and maintaining the dissolution stability of Ag.